An example of ball screws including a seal with slits is a ball screw disclosed in PTL 1. The seal provided in the ball screw is a contact seal having a cylindrical shape and has a seal mountain that is engaged with a helical groove of a screw shaft on an inner periphery thereof. The seal is mounted in a nut in such a manner that a part of the seal protrudes outward from an end face of the nut. Additionally, the protruding part is split by a plurality of slits. The length of the slits is made to be 0.7 times or more than a lead of the screw shaft.
Herein, the ball screw disclosed in PTL 1 satisfies a relationship of 0<θ1≤θ, where an angle between the slits extending in an axial direction and a screw shaft line is θ1 and a lead angle of the screw shaft is θ. Additionally, the slits are inclined by θ2 with respect to a vertical direction on a circumferential surface of the shaft. Foreign matter entering from a direction outside the nut hits against an inner peripheral surface of the seal contacted with the shaft, climbs up in a direction of the angle θ2, and is scraped out onto an outer peripheral surface of a seal trunk. After that, the foreign matter accumulated to some extent naturally falls by its own weight from the outer peripheral surface of the seal trunk.